A method well known in the art for measuring magnetic field is to utilize the Hall effect, which generates an electrical potential in a conductive material. The potential generated is directly dependent on an electric current flowing in the material and on the magnetic field perpendicular to the current.
FIG. 1 is a diagram of a ferromagnetic conductor 10, having a general rectangular film-like shape, as is known in the art. Conductor 10 has a current I flowing between faces 12 and 14 of the conductor, and there is a magnetic field B applied through faces 16 and 18 of the conductor, causing a magnetization M in the conductor. Face 16 (or 18) defines a plane of conductor 10. Field B acts on moving current carriers in conductor 10 to generate a Hall voltage VH between faces 20 and 22 of the conductor. In general:VH=I·(Rn·B+Re·M)  (1)
where Rn is a first constant, termed the normal Hall coefficient, and Re is second constant, termed the extraordinary Hall coefficient, for conductor 10.
The normal Hall coefficient, Rn, represents the effect of Lorentz forces on the current carriers in conductor 10. The extraordinary Hall coefficient Re, characteristic of conductors which are ferromagnetic, represents the effect of scattering of electrons in the presence of magnetic polarization.
In bulk ferromagnetic materials, Re can be much larger than Rn, so that for values of B lower than those saturating conductor 10, equation (1) can be rewritten as:VH=I·Re·M=I·Re·χB  (2)
where χ is an effective susceptibility, dependent on the geometry and composition, of conductor 10.
FIG. 2 is a graph, as is known in the art, illustrating a relation between a measured Hall resistance RHall and magnetic field B at room temperature, for a nickel film having a thickness of 100 nm. RHall corresponds to the term Re·χB of equation (2). The graph shows that in a region between −0.3 T and 0.3 T RHall varies approximately linearly with magnetic field, that the slopes of the linear sections,
            ⅆ              R        Hall                    ⅆ      B        ,are approximately 30 mΩ/T, and that there is a hysteresis of approximately 1.1•10−2 T. The term
      ⅆ          R      Hall            ⅆ    B  is termed the field sensitivity, F, of the film.
From equation (2), and the definitions Hall resistance RHall and of field sensitivity F,RHall=Re·χ·B, so that
                                          ⅆ                          R              Hall                                            ⅆ            B                          =                  F          =                                    R              e                        ·            χ                                              (        3        )            
Returning to FIG. 1, a sensitivity S of conductor 10, when it is used as a ferromagnetic Hall sensor, may be defined as
            V      H        B    ,so that from equations (2) and (3):
                    S        =                                            V              H                        B                    =                                    I              ·              χ              ·                              R                e                                      =                          I              ·              F                                                          (        4        )            
For Hall sensors which are not ferromagnetic, such as semiconductors, equation (1) becomes:VH=I·Rn·B  (5)
A sensitivity S, from equation (5), may be written as:
                    S        =                                            V              H                        B                    =                      I            ·                          R              n                                                          (        6        )            
Hall sensors using both ferromagnetic and non-ferromagnetic materials, the latter typically being semiconductors, are known in the art. Typically, an effective Hall coefficient for a bulk ferromagnetic, χ·Re, is significantly smaller than the Hall coefficient, Rn, of a semiconductor.
A relationship between the extraordinary Hall coefficient Re and the resistivity ρ of ferromagnets is known in the art. The relationship is of the form:Re∝ρn  (7)
where n is a constant.
The value of n in equation (7) is dependent on the composition of the ferromagnet, and typically lies in a range between 1 and approximately 4. Variations of ρ of the order of ten percent are typically produced by doping or temperature changes.
Ferromagnetic conductors such as conductor 10 may be implemented in one of two anisotropic forms. Planar anisotropy, wherein a direction of easy magnetization of the conductor lies in the plane of the conductor, and perpendicular anisotropy, wherein the direction of easy magnetization is perpendicular to the plane of the conductor. As known in the art, both forms exhibit some hysteresis, although the hysteresis of conductors which have perpendicular anisotropy is typically larger than the hysteresis of planar anisotropy conductors. Films with reduced thickness typically have planar anisotropy, although perpendicular anisotropy is known in such films, and in certain alloys, such as Co—Pt, Co—Cr, and Co—Cr—Ta. The implementation of conductor 10 as a planar or as a perpendicular anisotrope is typically a function of how conductor 10 is formed, and the composition of the conductor, as is known in the art.
An article titled “Spin-dependent scattering in weakly coupled nickel films,” by Gerber et al., in Europhysics Letters, 49(3), (2000), which is incorporated herein by reference, describes a process of forming thin films from ferromagnetic materials. Initially, as films are formed on an insulating substrate by electron beam evaporation or radio-frequency sputtering, islands of metal build on the substrate. The process of island formation continues until a conductance percolation threshold is achieved, wherein there is a conducting path through the film between weakly coupled islands of the film. If deposition of the ferromagnetic material continues, a ferromagnetic percolation threshold is achieved, wherein the film becomes ferromagnetic. Between the conductance percolation threshold and the ferromagnetic percolation threshold the film behaves substantially as a super-paramagnetic.
It will be appreciated that the conductance percolation threshold may be evaluated for substantially any film of conducting material, for example, by determining during formation of the film a point at which the film begins to conduct. Similarly, the ferromagnetic percolation threshold may be evaluated for substantially any film of ferromagnetic material, for example, by determining during formation of the film the point at which the film begins to behave as a ferromagnet. Alternatively or additionally, the conductance percolation threshold may be evaluated by indirect measurement of conductance of the film, and the ferromagnetic percolation threshold may be evaluated by determining the presence of hysteresis in the film. Other methods for determining both thresholds will be apparent to those skilled in the art.
U.S. Pat. No. 4,393,427, to Sakurai, whose disclosure is incorporated herein by reference, describes a magnetic detecting head comprising a Hall element. The Hall element is an amorphous ferromagnetic film comprising a rare earth/transition metal alloy, and having a thickness of 200 nm or more. U.S. Pat. No. 4,420,781, to Sakurai, whose disclosure is incorporated herein by reference, also describes a magnetic detecting head comprising a Hall element having a thickness of about 150 nm.
U.S. Pat. No. 5,206,590, to Dieny, et al., whose disclosure is incorporated herein by reference, describes a magnetoresistive sensor comprising a first ferromagnetic film and a second ferromagnetic film separated by a non-magnetic metallic film. The sensor uses a “spin valve” effect occurring between the two ferromagnetic films, wherein the resistance between two uncoupled ferromagnetic layers varies as the cosine of the angle between magnetizations of the two layers.
U.S. Pat. No. 5,361,226, to Taguchi, et al., whose disclosure is incorporated herein by reference, describes a magnetic memory comprising a ferromagnetic film which has perpendicular anisotropy. One of the embodiments of the memory comprises a film having a thickness of 50 nm.
U.S. Pat. No. 5,617,071, to Daughton, whose disclosure is incorporated herein by reference, describes a magnetoresistive layered structure having a plurality of layers of ferromagnetic films. Providing multiple layers increases a “giant magnetoresistive” (GMR) response of the structure when it is used as a field sensor.
U.S. Pat. No. 5,652,445, to Johnson, whose disclosure is incorporated herein by reference, describes a hybrid Hall device. The device comprises a ferromagnetic film which is over-layered on a portion of a conductive layer. The device generates an electric signal responsive to a fringe magnetic field, from the ferromagnetic film, through the conductive layer.
Notwithstanding the systems described above, there is a need for a relatively simple magnetic field sensor which may be simply and robustly fabricated.